Offshore drilling rigs are widely used in the exploration and exploitation of hydrocarbon reservoirs under the sea floor.
One type of drilling structure is the semi-submersible drilling rig that typically obtains its buoyancy from ballasted, watertight pontoons located below the ocean surface and wave action. The operating deck can be located high above the sea level due to the high stability of the design, and therefore the operating deck is kept well away from the waves. Structural columns connect the pontoons and operating deck. Other examples of offshore drilling rigs include drillships.
U.S. Pat. No. 6,766,860 discloses an offshore drilling rig of the semi-submersible type comprising two load paths within the same derrick. The derrick floor is elevated above the rest of the drilling deck. Rotary tables are positioned in the drill deck below the primary and secondary hoisting paths. On the drill deck, drill pipe and the drill bit is made up and run through the water column to the sea bed where it is rotated by either the rotary table and/or a rotating mechanism (top drive) suspended in the derrick. Later, casing tubulars are assembled in one of the hoisting paths and run into the hole. Ramps feed pipes to the primary and secondary hoisting paths respectively.
During drilling operations, auxiliary equipment may have to be lowered through the well centre. Examples of such auxiliary equipment may include logging-while-drilling equipment, measuring-while-drilling equipment, coiled tubing equipment, etc. and similar equipment other than the tubulars making up the drill string, well casing/lining or risers and other than heavy subsea equipment such as BOPs and Christmas trees. It is generally desirable to provide an offshore drilling rig that allows for a more efficient and flexible operation. It is further generally desirable to provide an offshore drilling rig that facilitates operation with a high degree of safety. More particularly, it is desirable to operate auxiliary equipment and similar suspendable equipment in a safe and efficient manner.